Optics(12th Grade > Physics ) Questions and answers for exam Preparation

12th Grade > Physics

OPTICS MCQs

Wave Optics, Ray Optics Refraction, Ray Optics Fundamentals, Ray Optics Curved Surface Refraction, Ray Optics Curved Mirrors

Total Questions : 111 | Page 3 of 12 pages

Question 21. The focus is defined for a concave mirror but not for a convex mirror because :

A concave mirror converges the rays to the focus, while a convex mirror diverges the rays.
Rays after striking a convex mirror never meet again
A convex mirror is not a focusing mirror
Trick question! The focus is defined for a convex mirror as the point where the reflected rays appear to meet.

Discuss Question

Answer: Option D. -> Trick question! The focus is defined for a convex mirror as the point where the reflected rays appear to meet.
:
D
The design of a convex mirror is in such a way that parallel rays of light after reflection from the convex mirror, diverge. These rays do not converge at a point as in the case of a concave mirror. But if we extrapolate the reflected rays, we can say they appear to meet at a point on the non-reflecting side of the mirror. We actually define this point as focus of a convex mirror.

The Focus Is Defined For A Concave Mirror But Not For A Conv...

Question 22. Sunlight, when it reaches the earth

is parallel.
can be considered approximately parallel.
is not considered approximately parallel.
just to solve questions we consider it to be parallel, in reality it is no were near the truth.

Discuss Question

Answer: Option B. -> can be considered approximately parallel.
:
B
Nope, sunrays are not exactly parallel. And no, we don't just randomly assume it to make problems and life easy! Rather, it is a good approximation to consider sunrays to be parallel when they reach us, as the sun is really really far (149.6 million km!) from the Earth, hence the angle between the rays are very very small. Go watch the video again to realize this.

Question 23. A convex lens of focal length f is placed somewhere in between an object and a screen. The distance between the object and the screen is x. If the numerical value of the magnification produced by the lens is m, then the focal length of the lens is

mx(m+1)2
mx(m−1)2
(m+1)2mx
(m−1)2mx

Discuss Question

Answer: Option A. -> mx(m+1)2
:
A

A Convex Lens Of Focal Length F is Placed Somewhere In Betw...

Question 24. A hollow double concave lens is made of very thin transparent material. It can be filled with air or either of two liquids

L_{1}

and

L_{2}

having refractive indices

n_{1}

and

n_{2}

respectively

(n_{2} > n_{1} > 1)

. The lens will diverge a parallel beam of light if it is filled with

Air and placed in air
Air and immersed in L1
L1 and immersed in L2
L2 and immersed in L1

Discuss Question

Answer: Option D. -> L2 and immersed in L1
:
D

\frac{1}{f} = (\frac{n_{2}}{n_{1}} - 1) (\frac{1}{R_{1}} - \frac{1}{R_{2}})

Where

n_{2}

and

n_{1}

are the refractive indices of the material of the lens
and of the surroundings respectively. For a double concave lens,

(\frac{1}{R_{1}} - \frac{1}{R_{2}})

is always negative.

A Hollow Double Concave Lens Is Made Of Very Thin Transparen...

Hence f is negative only when

n_{2} > n_{1}

Question 25. If in a plano-convex lens, the radius of curvature of the convex surface is 10 cm and the focal length of the lens is 30 cm, then the refractive index of the material of lens will be
[CPMT 1986; MNR 1988; MP PMT 2002; UPSEAT 2000]

1.5
1.66
1.33
3

Discuss Question

Answer: Option C. -> 1.33
:
C

Question 26. An object is placed at a distance of

\frac{f}{2}

from a convex lens. The image will be formed:

At −f, virtual and double its size.
At f, real and inverted.
At −2f, virtual and erect.
At 2f, real and erect.

Discuss Question

Answer: Option A. -> At −f, virtual and double its size.
:
A
Given

u = \frac{- f}{2}

\frac{1}{f} = \frac{1}{v} - \frac{1}{u}

\Rightarrow \frac{1}{f} = \frac{1}{v} + (\frac{1}{\frac{f}{2}}) \Rightarrow \frac{1}{v} = \frac{1}{f} - \frac{2}{f}

\Rightarrow \frac{1}{v} = \frac{- 1}{f}

v = - f

i.e., the image will be formed at focus of the lens on the same side as that of the object.
Now magnification

m = \frac{v}{u} = \frac{- f}{\frac{- f}{2}} = + 2

So the image formed will be virtual with magnification +2.

Question 27. The focal length of a concave mirror is 50cm Where an object be placed so that its image is two times and inverted

75cm
72cm
63 cm
50 cm

Discuss Question

Answer: Option A. -> 75cm
:
A

The Focal Length Of A Concave Mirror Is 50cm Where An Objec...

Question 28. An object 5 cm tall is placed 1 m from a concave spherical mirror which has a radius of curvature of 20 cm. The height of the image is